Cathodic protection systems may be monitored by permanent or portable reference cells. Portable reference cells may provide readings of questionable reliability since the results are dependent on a variety of factors such as cell placement and surface soil conditions. It requires an experienced and skilled technician to obtain reasonably reliable readings.
A permanent reference cell is more reliable but requires installation in the ground or electrolyte spaced from the buried or submerged structure. The cell is connected to a test station which must be accessible, i.e. normally above ground or not submerged. The test station is also electrically connected to the structure enabling the potential measurement to be taken conveniently at the test station.
A permanent reference cell must have a long design life and be relatively easy to install. Moreover, the installation procedure should itself avoid damage to the cell or test station. The test station should also be simple and easy to use. Over a long design life, the test station may itself be subject to corrosion, or more likely vandalism if caps can be removed or doors opened to provide access to the particular wiring connections. This is particularly true if the wiring connections provide movable or removable parts such as binding posts or even banana jacks.
Conventionally, an installing contractor orders a reference cell, a test station or terminal box, and sufficient wire to connect the cell to the test station and the test station to the buried structure or structures. The wiring connections are done in the field at what is usually a major construction site involving excavation, excavation machinery, backfills, backfill machinery and sometimes the removal and installation or replacement of pavement. This is all usually accomplished in an environment of mud or dirt. Before the system is backfilled, the system is tested. Complete aggravation results when the system is tested after installation and backfill and fails to work properly. Failure most often occurs at the electrical connections which are made in the field and subject to the visits of a construction site. Examples of cells commercially available are a variety of cells sold under the trademark PERMACELL.RTM. by Harco Corporation of Medina, Ohio. These include underground copper-copper sulphate, or silver-silver chloride cells, and silver-silver chloride cells for salt water applications. A variety of test stations are also available usually requiring removable caps, doors, etc., and binding post or banana jack wiring connections.